Process for producing magnesium utilizing aluminum metal reductant

ABSTRACT

Magnesium metal is produced by the reaction of aluminum metal with a calcium magnesium aluminate slag or with magnesium oxide in the presence of such slag, wherein the silicon dioxide content of the slag is maintained at five weight percent or less. An excess of aluminum metal relative to the amount of magnesium oxide is employed thereby obtaining a high conversion of the magnesium oxide to magnesium metal.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 610,430 filed onSept. 4, 1975 now U.S. Pat. No. 4,033,759.

BACKGROUND OF THE INVENTION

The present invention relates to the production of magnesium metal bythe reduction of magnesium oxide at elevated temperatures in thepresence of an aluminum metal reducing agent and a molten oxidic slag,in an electric furnace, and the condensation of vaporized magnesium in acondenser.

An advantageous method of producing magnesium lies in the chemicalreduction of magnesium oxide with a reducing agent in the presence of amolten oxide slag, in an electric furnace. A variety of metallothermicprocesses, as they are commonly called, for the production of magnesiumemploying various reducing agents, various types of reactants, and undervarious conditions of temperatures and pressures have been proposed.

In general, the various metallothermic processes are concerned with theproduction of metallic magnesium by the reduction of magnesium oxidewith a metal reductant at elevated temperatures. Magnesium oxide,usually in the form of calcined dolomite (dolime) or calcined magnesiteor mixtures thereof, is caused to react with a metallic reducing agent,such as silicon, aluminum, calcium or mixtures or alloys thereof, in thepresence of a molten slag bath in a furnace at temperatures in excess of1300° C, to release magnesium vapor which may be condensed andcollected. Some of the processes are carried out in the presence of aninert gas.

An early process of this type, called the Pidgeon process provides forthe production of magnesium by the reduction of magnesium oxide withferrosilicon.

A more recent process is exemplified by U.S. Pat. 2,971,833 and is knownas the Magnetherm process. The process is operated under a very highvacuum (pressure ranging between 5 and 20 millimeters of mercury) and ata temperature of about 1500° C utilizing an electric furnace. Silicon isemployed as the reductant, preferably in the form of ferrosiliconcontaining 70-80 percent Si, silicon of a purity in excess of 97%, or analuminum-ferrosilicon. Care is taken to avoid that the silicon contentof the residual ferrosilicon drops below 33.5%. A magnesia containingsubstance is dissolved in a liquid slag consisting essentially of lime,silica and alumina wherein the ratio of calcium oxide to silicon dioxideand the ratio of aluminum oxide to silicon dioxide are controlled.

Processes employing aluminum metal as the reductant are exemplified byU.S. Pat. Nos. 3,782,922; 2,527,722 and 2,527,724. The first patentdiscloses the production of magnesium by the reduction of magnesiumoxide from a mixture of magnesium oxide and calcium oxide with areducing agent comprising at least 85% aluminum in the presence of amolten calcium-aluminate slag bath at a temperature of about 1300°-1700°C and a pressure of about atmospheric. Slag is removed when themagnesium oxide content thereof is less than five percent, with the slagcomprising 35-65 percent aluminum oxide, 35-55 percent calcium oxide and0-10 percent silicon dioxide. The other two patents utilize magnesiumsilicate in pure or ore form and aluminum as reactants. Dolomite is alsoused in the former and the magnesium silicate is of a particulargranular size in the latter.

U.S. Pat. No. 3,658,509 also discloses the use of aluminum as thereductant, as well as silicon and aluminum-silicon alloys with thelatter being preferred. In such process, an inert gas is used to obviateat least in part the need of a high vacuum, and the slag contains 20-50percent silicon dioxide.

Copending application Ser. No. 610,430, now U.S. Pat. No. 4,033,759discloses the use of aluminum metal as a reductant in such a manner asto produce a slag wherein, by weight percent, calcium oxide ranges from30-65, aluminum oxide ranges from 28-64, magnesium oxide ranges from6-13 and silicon dioxide is equal to or less than 5.

The primary object of this invention is to provide an improved processfor production of magnesium metal wherein aluminum metal is reacted witha calcium magnesium aluminate slag, which eliminates the disadvantagesof the prior art processes, while retaining the benefits thereof.

Another object of this invention is to provide a process for productionof magnesium metal at atmospheric pressure by reaction of aluminum metalwith a calcium magnesium aluminate slag, or with magnesium oxide in thepresence of such slag, wherein the silicon dioxide content of the slagis maintained at a low level.

Another object of the invention is to provide a process for producingmagnesium metal utilizing aluminum metal as the reductant, whereinmagnesium vapor is produced at a pressure of one atmosphere therebyeliminating the need for vacuum equipment and permitting a continuousoperation at such normal pressure.

An important object of the present invention is to provide a process ofthe foregoing type which is economical and which can utilize aluminummetal scrap as the metal reductant.

Other objects and advantages of the present invention will become morereadily apparent from a consideration of the description hereinafter.

SUMMARY OF THE INVENTION

The instant invention relates to a process for the production ofmagnesium metal by reaction of a calcium magnesium aluminate slag or ofmagnesium oxide in the presence of such slag, with aluminum metal ofabout 80 percent or greater purity at reasonable pressures, preferablyabout atmospheric pressure, and at elevated temperatures of about1500°-1700° C, preferably about 1550°-1600° C. Magnesium evolves as avapor and is condensed and collected in suitable apparatus. Theconcentration of silicon dioxide in the slag is kept at a low value,about five weight percent or less, preferably two weight percent orless.

An excess of aluminum metal relative to the amount of magnesium oxide isemployed thereby obtaining a high conversion of the magnesium oxide tomagnesium metal.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the process of the present invention, aluminum metal is reacted witha calcium magnesium aluminate slag, or with magnesium oxide in thepresence of such slag to produce magnesium metal. The process isillustrated by the following equations:

    1.12 CaO + 3.0 MgO + (2 + x)Al + Slag → CaO.Al.sub.2 O.sub.3 + 0.12 CaO + 3 Mg ↑ + xAl + slag

wherein, x = excess Al used and ranges

from 0.1-2.0 and preferably

from 0.1-0.2

The reduction reaction is carried out in an internally heated electricfurnace at a temperature of about 1500°-1700° C, with 1550°-1600° Cbeing preferred and about 1550° C being most preferred and at a pressureof about 0.5-2.0 atmospheres, preferably about atmospheric pressure. Theconcentration of the various slag components is controlled. Silicondioxide is kept at a low level, no greater than five weight percent andpreferably about two weight percent or less. As the reaction continues,MgO in the slag is consumed by reaction with the aluminum metal.Additional MgO or dolime (T CaO.sup.. MgO, where 0.5 ≦ T ≦ 2.0) is addedas necessary to maintain the desired slag composition as will be morefully explained hereinafter. It is necessary however that the silicondioxide content of the slag be kept at five weight percent or less.Materials added to the slag should also contain no more than 5 weightpercent SiO₂.

It has been found to be advantageous to operate with a reducing agentcontaining 80 weight percent or more aluminum metal, to restrict silicondioxide content of feed materials and slag to 5 weight percent or less,and to use an excess of aluminum metal reductant such that slag in thereactor and slag tapped or withdrawn from the reactor contain 6-16weight percent magnesium oxide. This combination of reactants provides asuperior process, one in which the reaction between aluminum metal andmagnesium readily occurs, one in which a high utilization of aluminummetal is obtained as the excess aluminum can be recovered and recycled,and one in which a high conversion of magnesium oxide to magnesium metalis obtained. The advantages of using these conditions will become moreapparent from the discussion in the following paragraphs.

Preferably, the various reactants are added in their respective requiredamounts simultaneously and at a sufficiently slow rate to permit thedesired reaction to take place. A deficiency of magnesium oxide ratherthan an excess thereof has been unexpectedly found to produce superiorresults.

Aluminum is an active metal, and reacts at room temperature with avariety of acids, bases, and other chemical reagents. Its chemicalreactivity extends also to the higher temperatures required for theproduction of magnesium metal, and in high temperature systems it is asuperior reducing agent in comparison with less active metallic reducingagents, such as ferrosilicon or silicon metal.

Aluminum metal is a more active reducing agent, in that it produces ahigher vapor pressure of the desired magnesium product at a lowertemperature than other known reducing agents. The usual method ofremoving magnesium from such high temperature reaction systems is as avapor, and it is subsequently condensed to a liquid or solid forrecovery. Aluminum metal is superior as a reducing agent to less activemetals such as ferrosilicon or silicon.

In addition to using aluminum metal as a reducing agent, it is alsoessential to keep the concentration of silicon dioxide in the reactionat a low value, at least less than or equal to five weight percent.Because of its high reactivity, aluminum metal at high temperaturesreacts not only with magnesium oxide, but also with silicon dioxide andother siliceous materials. Silicon metal is the product from reductionof silicon dioxide or siliceous materials, rather than the desiredmagnesium metal. Thus when silicon dioxide or other siliceous materialsare present in the reactor, part of the expensive aluminum reducingagent will be converted to a by-product, namely silicon metal, ratherthan the desired magnesium metal product. Although it is possible toforce the co-produced silicon metal to react with magnesium oxide in thereactor to yield magnesium metal, to do so requires the application ofhigher temperatures and lower pressures than when aluminum metal isreacted directly with magnesium oxide in the absence of siliceousmaterials. Silicon dioxide concentration in the reactor and of incomingfeed materials must be restricted to five weight percent or less.

The rate and completeness of a chemical reaction are dependent on theconcentrations of the reactants. Thus, in the reaction:

    1.12 CaO + 3.0 MgO + (2 + x) Al → CaO.Al.sub.2 O.sub.3 + 0.12 CaO + 3 Mg↑

the rate of magnesium production and the completeness of the reactionare dependent on the concentrations of aluminum metal and magnesiumoxide in the reactor. The high concentration of aluminum metal employedin this invention facilitates reaction. The amount of aluminum metalused or fed into the reactor ranges from 105 percent to 200 percent ofthe stoichiometric amount required to react with the magnesium oxide fedinto the reactor. The excess amount of aluminum metal is thus fivepercent to 100 percent of the stoichiometric amount of the aluminumrequired to react with the magnesium oxide. The concentration ofmagnesium oxide in the reactor and in the tapped slag must be limited toavoid increasing the melting point of the slag to too high a value andto avoid excessive loss of magnesium oxide in the tapped slag. Themagnesium oxide concentration range of 6-16 weight percent inclusive, inthe reactor, and in the tapped slag satisfies the need for completenessof reaction while avoiding excessive concentrations of magnesium oxidein the slag.

Although aluminum metal used in the process should have a purity equalto or greater than 80 weight percent and preferably greater than orequal to 90 weight percent, most aluminum scrap is suitable. Metallicimpurities in the aluminum, such as iron, manganese and silicon can betolerated. Such impurities are inert under the process conditions andcan be withdrawn from the furnace or reactor from time to time, forexample, when the slag is tapped. Non-metallic impurities in thealuminum, for example, aluminum oxide can also be tolerated. The totalof these non-metallic impurities other than Al₂ O₃, MgO or CaO shouldnot exceed five weight percent. The aluminum metal should also be driedbefore use and should contain no more than 0.25 weight percent water.

The oxides and other compounds making up the slag composition are asfollows by weight percent:

    ______________________________________                                                         Range Preferred Range                                        ______________________________________                                        CaO                30-65   33-62                                              Al.sub.2 O.sub.3   28-64   32-62                                              MgO                 6-16    6-10                                              SiO                ≦5                                                                             ≦2                                          Other oxides, halides, sulfides                                                                  ≦5                                                                             ≦2                                          ______________________________________                                    

Dolime, CaO or MgO may be added to the slag separately or in combinationas needed to maintain the desired slag composition. As stated before,the dolime need not contain equal amounts of CaO and MgO, but maycontain T moles of CaO per mole MgO, e.g., T CaO.sup.. 1 MgO, where 0.5≦ T ≦ 2.0. These oxides should also contain not more than 5 weightpercent SiO₂ and preferably not more than 2 weight percent. Such oxidesshould also be substantially free of H₂ O and CO₂ before use and shouldcontain not more than 0.5 weight percent H₂ O and 0.5 weight percentCO₂. Additional MgO is added as necessary to maintain 6-16 weightpercent MgO in the slag.

The reaction should be carried out at a temperature of about 1500°-1700°C, preferably about 1550°-1600° C and at a pressure range of about0.5-2.0 atmospheres, preferably about 1.0 atmosphere.

In carrying out the invention, a slag of a composition within theforegoing ranges is prepared and melted. The various slag ingredients,CaO, MgO, dolime, and Al₂ O₃ may be mixed together or a slag of asuitable composition form a previous operation may be used. Anatmosphere of inert gas, such as argon or hydrogen is provided duringmelting. Heat is supplied for melting by suitably locating electrodes soas to pass a current through the slag, or by any other suitable means.After the desired temperature of the molten slag is achieved, a streamof aluminum particles is charged into the slag. Simultaneouslytherewith, an oxide feed stream containing MgO and dolime is added tothe slag to keep its composition from changing. Alternatively, thealuminum metal and oxide feed may be intermingled, and added to thereactor as a mixture. Magnesium vapor is evolved from the surface of theslag, is conducted to a suitable condenser, and is condensed at apressure of one atmosphere. An inert gas such as argon or hydrogen isused to prevent air from contacting the magnesium. However, the inertgas does not flow continuously from reactor to condenser, and hence doesnot transport magnesium metal vapor from reactor to condenser. As thereaction proceeds, the slag level in the reactor increases. From time totime, a portion of the slag and any unreacted inert metals such as iron,silicon, etc. are removed through a suitable tap hole in the reactor.

Examples

The examples described in the following table are illustrative of theprocess, and are carried out as described hereinbefore. The reactor iskept at a temperature of 1550° C ± 10° C and at about atmosphericpressure. Reactants and other oxides are added to the reactor eitherseparately, or as mixtures. MgO is added both as the individual oxideand as dolime. The reaction which occurs in the reactor yields magnesiummetal vapor and slag. The magnesium metal vapor flows to a condenser,and is condensed to a liquid or solid. Slag remains in the reactor andis removed from time to time.

    __________________________________________________________________________    Feed                     Products                                             Reactants and Other Oxides - Moles                                                                     Moles                                                                              Grams                                                                              Slag Composition                                        Dolime  Al  Mg   Slag wt %                                       Example                                                                            MgO CaO MgO CaO Metal                                                                             Metal                                                                              Produced                                                                           MgO                                                                              Al.sub.2 O                                                                        CaO                                 __________________________________________________________________________     1*  0.68                                                                              2.08                                                                              1.46                                                                              1.46                                                                              1.9 1.4  406  10 45  46                                  2    1.88    1.10                                                                              1.10                                                                              2.2 2.4  143  16 52  32                                  3    1.88    1.10                                                                              1.11                                                                              2.4 2.7  178  7  59  34                                  4    1.88    1.10                                                                              1.11                                                                              2.9 2.7  174  8  58  34                                  __________________________________________________________________________     *Control Sample - Following Teachings of Serial No. 610,430              

From the examples, it is readily seen that an excess of aluminum metalin accordance with the instant invention produces substantially highyields of magnesium metal.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof and various changes may be madewithin the scope of the appended claims without departing from thespirit of the invention.

What is claimed is:
 1. A process for production of magnesium in areaction-condensation system having a reducing furnace zone and acondensation zone, comprising reacting in the reducing furnace zonealuminum metal containing at least 80 weight percent aluminum and acalcium magnesium aluminate slag or magnesium oxide in the presence ofsaid slag containing not more than five weight percent silicon dioxide,the amount of aluminum metal fed into the reducing furnace zone being inexcess of the stoichiometric amount of aluminum required to react withthe magnesium oxide fed into the reaction zone, and slag removedtherefrom contains about 6-16 weight percent magnesium oxide, carryingout the reaction at a temperature of about 1500°-1700° C at a pressureof about 0.5-2.0 atmospheres, maintaining the silicon dioxide contentand the magnesium oxide content of the molten oxidic slag in thereaction zone at least as low as five weight percent and about 6-16weight percent, respectively, evolving magnesium vapor from the reactionzone to the condensation zone, and condensing and recovering themagnesium as a product.
 2. The process of claim 1, wherein the excessamount of aluminum metal fed into the reducing furnace zone is from fivepercent to 100 percent of the stoichiometric amount of aluminum requiredto react with the magnesium oxide fed into the reaction zone.
 3. Theprocess of claim 1, wherein the excess amount of aluminum metal fed intothe reducing furnace zone is from 5 percent to 10 percent of thestoichiometric amount of aluminum required to react with the magnesiumoxide fed into the reaction zone.
 4. The process of claim 1, wherein thereaction is carried out at a temperature of 1550°-1600° C.
 5. Theprocess of claim 1, wherein the reaction is carried out at a pressure ofabout one atmosphere.
 6. The process of claim 1, wherein the silicondioxide content of the molten oxidic slag is maintained at least as lowas two weight percent.
 7. The process of claim 1, wherein said aluminumhas a purity of at least 90 weight percent.
 8. The process of claim 1,wherein as the reaction continues, and as magnesium oxide in the slag isconsumed by reacting with the aluminum, additional oxides are added asnecessary to maintain the composition of the slag in weight percent asfollows:

    ______________________________________                                        calcium oxide (CaO)    30 - 65                                                aluminum oxide (Al.sub.2 O.sub.3)                                                                    28 - 64                                                magnesium oxide (MgO)   6 - 16                                                silicon dioxide (SiO.sub.2)                                                                          ≦5                                              and other oxides, halides, sulfides                                                                  ≦5                                              ______________________________________                                    


9. The process of claim 8, wherein the magnesium oxide is in the form ofcalcined dolime of the formula T CaO.sup.. MgO where 0.5 ≦ T ≦ 2.0. 10.The process of claim 1, wherein an inert gas is employed to prevent airfrom contacting the magnesium.
 11. In a process for producing magnesiumby reaction of magnesium oxide with aluminum in a reaction-condensationsystem and utilizing a molten oxidic slag, the improvement therein,wherein the aluminum has a purity of at least 80 weight percent, theamount of aluminum metal fed into the reaction zone is from 5 to 100percent in excess of the stoichiometric amount of aluminum required toreact with the magnesium oxide, the silicon dioxide content and themagnesium oxide content of the molten oxidic slag is maintained at lessthan about five weight percent of said slag and about 6-16 weightpercent of said slag, respectively.
 12. The process of claim 11, whereinthe silicon dioxide content of the molten oxidic slag is maintained atless than about two weight percent of said slag.
 13. A process for theproduction of magnesium metal in a reaction-condensation system having areducing furnace zone and a condensation zone, comprising preparing andmelting in the reaction zone a calcium magnesium aluminum oxide slag ofthe following weight percent composition:

    ______________________________________                                        Calcium oxide (CaO)    30 - 65                                                Aluminum oxide (Al.sub.2 O.sub.3)                                                                    28 - 64                                                Magnesium oxide (MgO)   6 - 16                                                Silicon dioxide (SiO.sub.2)                                                                          ≦5                                              ______________________________________                                    

reacting with the molten oxidic slag in the reaction zone an aluminummetal of at least 80 weight percent purity, the amount of the aluminummetal being from five percent to 100 percent in excess of the amount ofaluminum required to react stoichiometrically with the magnesium oxide,carrying out the reaction at a temperature of about 1500°-1700° C and ata pressure of about 0.5-2.0 atmospheres, periodically withdrawing moltenoxidic slag from the reaction zone, maintaining the silicon dioxidecontent and the magnesium oxide content of the molten oxidic slag in thereaction zone and the molten oxidic slag withdrawn therefrom at fiveweight percent or less and about 6-16 weight percent, respectively,evolving magnesium vapor from the reaction zone to the condensation zoneand recovering magnesium metal as a product.
 14. The process of claim13, wherein the silicon dioxide content of said slag is maintained attwo weight percent or less.
 15. The process of claim 13, wherein saidtemperature is about 1550°-1600° C.
 16. The process of claim 13, whereinthe excess amount of aluminum is from 5 percent to 10 percent of thestoichiometric amount of aluminum.
 17. The process of claim 13, whereinsaid pressure is about one atmosphere.
 18. The process of claim 13,wherein the melting in the reaction zone is conducted in an atmosphereof an inert gas.
 19. The process of claim 13, wherein the aluminum metalis aluminum scrap.